Very close tolerances are sought between the spinning blades of the turbine section of a gas turbine engine and the circumscribing structure of the engine case. To achieve this, a portion of the engine case structure is surfaced with an abradable material. Such material generally remains intact, but is easily disintegratable when contacted by the spinning blade.
The abradable material is usually applied to small segments of metal, and in early engines, the abradable surfaces of the segments were made of relatively delicate metal, such as honeycomb or fiber metal. When the superalloy of turbine blades was insufficient in wear resistance, various hardfacing metals were applied.
But more recently, the demand for higher temperatures has led to the use of ceramic abradable surfaces on the static seals. Unfortunately, such materials are not so abradable as the metals they replace. And with the higher temperatures associated with ceramic seal use, the properties of the older metal turbine blade tips diminish. Not only do the high temperatures at turbine blade tips present wear problems, but the centripetal force associated with the high speed of blade spinning produces strains which can cause failure. Further, the cyclic temperature nature of the use can cause strains and failures associated with differential thermal expansions. Thus, resort was had to the use of composite metal-ceramic materials, such as the silicon carbide-nickel superalloy combination described in commonly owned U.S. Pat. No. 4,249,913 to Johnson et al.
As described in the Johnson patent, abrasive tips for turbine blades have been fabricated by pressing and solid state sintering of a mixture of metal and ceramic powders. Once made, the inserts are attached to the blade tip by brazing type processes. But both the manufacture of the abrasive tip material and adhering it to the tip have been difficult and costly.
The Johnson et al. type tips have performed well, and this is attributable to the uniform dispersion of ceramic in the metal matrix, a dispersion which is attainable by solid state processes.
But lower cost and higher performance alternatives have been sought, and these include plasma spraying and brazing type processes. Of course, conventional plasma spraying of a mixture of ceramic and metal has long been known, but such simple processes do not produce the requisite wear resistance and high temperature strength. Specialized plasma spray techniques have been developed, such as one in which a superalloy matrix is sprayed over previously deposited grits, followed by hot isostatic pressing. However, the technique is best used where only a single layer of particulate is sufficient.
And in both the Johnson et al. and the plasma spray processes, the grain size of the matrix is fine, a reflection of the fine grain powders. Fine grain size tends to limit creep strength at high temperature.
Fusion welding of ceramic and metal composites is not feasible with superalloy turbine blades since the substrate metal has a specialized metallurgical structure which is disturbed by the high temperature of fusion. A uniform deposit of metal and ceramic powders can be placed on a substrate through plasma spraying, or other powder metal techniques, such as are used to place brazing powders, and the deposit can then be heated to its temperature of fusion to consolidate such into a cast mass. However, it is found that doing such does not result in a uniform dispersion of ceramic in the matrix; the ceramic tends to go to the surface of the fused material due to buoyancy. In the critical applications like turbine blades, there must be achieved uniformity, to optimize the properties of the abrasive material, and minimize the weight which the turbine blade must carry.